In order to control the energy produced by a reactor core, various control rods or the like are insertable into the reactor core. Typically, as shown in U.S. Pat. Nos. 3,604,746 (Notari) and 3,853,699 (Frisch et al), a plurality of control rods are joined to a single drive rod. The single drive rod extends through the reactor vessel head in a pressure housing and a control drive mechanism is utilized to raise or lower the drive rod and the associated control rods. Thus, as all of the associated control rods are operated simultaneously, the relatively large worth of a single control rod assembly causes severe perturbations in the axial flux during movement. Therefore, part length control rods were utilized to trim the axial power distribution and these part length control rods were joined in assemblies with a suitable drive rod and control drive mechanism. However, this approach increases the number of pressure housing which must be located on the reactor vessel head.
Other prior art of interest includes U.S. Pat. No. 3,989,589 (Frisch, deceased et al) which provides for locating a plurality of drive rods in a single pressure housing. Each drive rod is attached to an appropriate control rod assembly and is hydraulically actuated to move the associated assembly.